Show task command + response/error preview on dashboard timeline hover

[vybe]

Summary

Add hover tooltips on the dashboard timeline view that surface a one-line preview of the initial task command and a one-line snippet of the response (or error text) for each execution, so it's possible to quickly scan what happened across executions without clicking into each one.

Context

The timeline view currently shows execution entries but lacks at-a-glance context — to see what an execution actually did, you have to click in. When triaging activity across many agents/schedules, that's slow. A hover tooltip with command + outcome preview turns the timeline into a scannable signal feed.

Acceptance Criteria

• Hovering over a timeline entry shows a tooltip with two lines:
  • Line 1: first line / first ~80 chars of the initial task command (truncated with ellipsis)
  • Line 2: first line / first ~80 chars of the response on success, or the error message on failure
• Failed executions clearly distinguish error text (e.g., color/prefix) from successful response previews
• Tooltip works for all relevant timeline entry types (chat, schedule executions, tool calls if applicable)
• No additional API round-trip on hover — preview text is included in the timeline payload (or already available from existing fields)
• Long content is truncated cleanly (no overflow, no broken markdown rendering)
• Mobile / small viewport behavior is acceptable (hover may degrade to tap/long-press or be omitted)

Technical Notes

• Frontend: dashboard timeline component (likely under src/frontend/src/views/ or a timeline-specific component); activity store at src/frontend/src/stores/
• Backend data: agent_activities.details (JSON) already carries action description; chat_messages.content and schedule_executions.response/error carry the body. Confirm whether the existing /api/activities/timeline payload includes enough preview data, or whether a preview field needs to be added server-side to avoid sending full message bodies.
• Keep tooltip component reusable — same pattern likely valuable in other list views.